The metal-oxide semiconductor field-effect transistor (MOSFET or MOS device) is a dominant and important device in fabricating memory devices and integrated circuits, and various types of MOSFETS are known. MOSFET technology includes NMOS, PMOS, and CMOS technology. NMOS and PMOS devices are n-channel and p-channel devices, respectively, and CMOS devices comprise n-channel and p-channel devices integrated on the same chip. Other acronyms used to identify MOSFETs include DMOS (wherein “D” stands for “diffusion” or “double diffusion”), IGBT (Insulated Gate Bipolar Transistor), BiCMOS (CMOS having bipolar devices), and DGDMOS (Dual Gate DMOS).
There is a continuing desire in the microelectronics industry to miniaturize device components, increase the circuit density in integrated devices, and lower the cost of making the devices to increase their availability to consumers (e.g. large emissive displays, electronic paper, smart cards, and so forth). One field of research has explored the configuration and materials used in traditional, inorganic semiconductors. In this area, for example, new materials have been developed for use in making active dielectric layers, i.e., high-dielectric strength materials to be used in place of thin films of a-SiOx. As the cell size has shrunk, designers have resorted to extremely thin or non-planar films of a-SiOx, but these films have been problematic as they exhibit a decreased reliability due to finite breakdown fields or have other attendant problems such as step coverage and conformality. Thus, recently attention has focused on thicker films of Ta2O5, TiO2, barium strontium titanate (BST) composites including (Ba,Sr)TiO3, and other materials including use of doped titanium and tantalum oxides. See, e.g., U.S. Pat. No. 5,912,797, titled “Dielectric Materials of Amorphous Compositions and Devices Employing Same,” issued Jun. 15, 1999 to inventors Schneemeyer and VanDover, and assigned to Lucent Technologies, Inc., the assignee herein, which is incorporated herein by reference.
Another area of research involves the development of new materials for use is in the insulating layers of inorganic semiconductors (e.g., as an insulator in a multilayer interconnection or an insulating layer between the semiconductor chip and substrate containing active components). In this quest, an opposite result is desired of obtaining low-dielectric constant materials. As may be appreciated, research relating to (I) the development of high-dielectric strength materials for use in active components, and (it) the development of low-dielectric materials for use in interconnect insulating layers, reflect distinct fields as the various processing and performance constraints differ in each case. Insulating properties relate to the ability of materials to block the passage of electrical charges, whereas dielectric properties relate to the ability of materials to store electrical charges. The current passage and current storage processes within a material are microscopically completely different. See, e.g., C. Kittel, INTRODUCTION TO SOLID STATE PHYSICS (John Wiley & Sons, NY). An increase in a material's insulating properties through chemical substitution may, in fact, decrease the dielectric properties.
In developing insulating materials for inorganic semiconductors, recently much attention has focused on forming highly porous organic polysilica materials comprising hybrid silsesquioxanes. See, e.g., U.S. Pat. No. 5,773,197 issued Jun. 30, 1998, “Integrated Circuit Device and Process for Its Manufacture,” and U.S. Pat. No. 5,895,263, issued Apr. 20, 1999, “Process for Manufacture of Integrated Circuit Device,” both of which are assigned to IBM Technologies Inc. and are incorporated herein by reference. These low-dielectric constant materials are used for fabricating intermediate layers between circuit layers, and they typically comprise reaction products of polysilicas and highly-branched or high molecular weight oligomers. High porosity is sought in making these materials to reduce the dielectric constant, advantageously to values of less than 3, more advantageously to values of less than 2.7 or 2.2.
The high porosity—and thus low dielectric constant—of these materials is achieved through various processing steps involving decomposition of the polymers to leave voids in the materials. To illustrate, the polymers are mixed in a solution with the polysilica; end groups of the polymers are reacted with the polysilica; the polymers are decomposed (e.g., by exposure to radiation or high temperatures, i.e. 350 to 400° C.); and then the polymers are allowed to diffuse out of the matrix, leaving voids in the polysilica material, generally the size of the domains of the decomposable polymer which can be controlled by molecular weight. (See, e.g., '263 patent, cited above, at cols. 34.) The resultant films typically have a desired thickness in the range of 0.2 to 1 μm, and in some instances less than 0.2 μm, making control of pore size, crack-resistance, and mechanical stability (e.g., modulus, hardness, toughness, etc.) significant considerations. See, e.g., Hawker et al., “Dendri-Glass—Design of Ultra-Low Dielectric Constant Materials Using Specialty Highly-branched Polymers,” Abstracts of Papers of ACS, Vol. 215 (1988), at p. 301; Cook et al., “Stress-Corrosion Cracking of Low Dielectric-Constant Spin-On Glass Thin Films,” Electrochemical Society Proceedings, Vol. 98-3, pp. 129-148; and Cook et al., “Properties Development During Curing of Low Dielectric-Constant Spin-On Glasses,” Mat. Res. Soc. Symp. Proc. Vol. 511 (1998), Materials Research Society, pp. 33-38, all three of which are incorporated herein by reference. Thus, the use of silsesquioxanes as low dielectric constant materials for insulating layers of inorganic semiconductor devices presents drawbacks in terms of the processing steps required to achieve suitable materials and conflicting optimization parameters.
Besides developing new materials for inorganic semiconductors, the drive toward hybridization and low-cost electronics has precipitated another area of research relating to the development of organic field-effect transistors (FETs). Organic materials are attractive for use in electronic devices as they are compatible with plastics and can be easily fabricated to provide low-cost, lightweight, and flexible devices with plastic substrates. However, organic devices provide their own materials constraints, e.g., concerns in developing active materials include their compatibility with and adhesiveness to plastic substrates and stability during processing steps. To illustrate, a dielectric film for an organic device should be formable at lower temperatures than those used in fabricating typical, inorganic devices, so that the films are compatible with a large number of plastics.
Presently-available organic polymers do not meet all the criteria needed for high-performance, low cost organic FETs. For example, polyimides are commonly used as dielectrics in organic devices. However, most polyimides require high curing temperatures above 200° C., rendering them unsuitable for use with many plastic substrates. Pre-converted soluble polyimides may be used to achieve a lower curing temperature but these usually have limited solubilities in fabricating thicker films (˜greater than 1 μm in thickness) which are necessary for the high voltages at which organic FETs operate (voltages of ˜>50 Volts).
Another organic material used as a dielectric in organic devices and suitable for spin-casting is benzocyclobutene (BCB), but this requires high conversion temperatures and strict exclusion of oxygen during curing. Also, when BCB is used as a dielectric, the p-channel devices tend to have high-off currents whereas n-channel devices tend to have low field-effect mobilities (e.g., as compared with devices fabricated with SiO2 as dielectrics). All-polymer logic circuits have been made incorporating poly(vinylphenol), but the current-voltage (I-V) characteristics of these devices tend to change dramatically with time. See, e.g., Z. Bao et al, “Printable Organic and Polymeric Semiconducting Materials and Devices,” J. MATER. CHEM. Vol. 9, (1999), at p. 1895. Poly(methylmethacrylates) (PMMA) are other organic materials that have been used as dielectrics in organic devices but these tend to have low glass transition temperatures and are easily soluble in other organic solvents used during device processing which is disadvantageous.
Furthermore, microcontact printing has evolved as an advantageous patterning method for producing feature sizes as small as 1 μm. While SiO2 as a dielectric is compatible with micro-contact printing, many of the polymers used for organic dielectrics are not, e.g., the polymers delaminate from their substrates when the micro-contact printing process is carried out.
As may be appreciated, those in the field of semiconducting devices continue to search for new materials and components to reduce the size, increase the efficiency, simplify the process, and reduce the cost of fabricating the devices. In particular, it would be advantageous in realizing high-performance field-effect transistors to provide solution processable dielectric materials compatible with organic semiconductors and with microcontact printing processes. These and further advantages may appear more fully upon considering the description given below.